LM3556_15 Datasheet, PDF (28/39 Pages) Texas Instruments – Synchronous Boost LED Flash Driver With High-Side Current Source

LM3556
SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015
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Larger capacitors (for example, a 22-µF capacitor) or capacitors in parallel may be used if lower output voltage
ripple is desired. To estimate the output voltage ripple, considering the ripple due to capacitor discharge (ΔV_Q)
and the ripple due to the capacitors equivalent series resistance (ESR) (ΔV_ESR), use Equation 1 and Equation 2.
For continuous conduction mode, the output voltage ripple due to the capacitor discharge is:
'V_Q =
I_LED x ⁽V_OUT - V_IN⁾
f_SW x V_OUT x C_OUT
(1)
The output voltage ripple due to the ESR of the output capacitor is found by:
'V_ESR
=
R _ESR
x
§^ILED
©
x V_OUT·
V_IN ¹
+ 'I_L
where
'I_L =
V_IN x ⁽V_OUT - V_IN⁾
2 x f_SW x L x V_OUT
(2)
In ceramic capacitors the ESR is very low so the assumption is that 80% of the output voltage ripple is due to
capacitor discharge and 20% from ESR. Table 16 lists different manufacturers for various output capacitors and
their case sizes suitable for use with the LM3556.
8.2.2.2 Input Capacitor Selection
Choosing the correct size and type of input capacitor helps minimize the voltage ripple caused by the switching
of the LM3556 device’s boost converter and reduces noise on the boost converter's input terminal that can feed
through and disrupt internal analog signals. In the Figure 41 a 10-µF ceramic input capacitor works well. It is
important to place the input capacitor as close as possible to the LM3556’s input (IN) pin. This reduces the series
resistance and inductance that can inject noise into the device due to the input switching currents. Table 16 lists
various input capacitors recommended for use with the LM3556.
Table 16. Recommended Input/Output Capacitors (X5R/X7R Dielectric)
MANUFACTURER
TDK Corporation
TDK Corporation
Murata
Murata
PART NUMBER
C1608JB0J106M
C2012JB1A106M
GRM188R60J106M
GRM21BR61A106KE19
VALUE
10 µF
10 µF
10 µF
10 µF
CASE SIZE
0603 (1.6 mm × 0.8 mm × 0.8 mm)
0805 (2 mm × 1.25 mm × 1.25 mm)
0603 (1.6 mm x 0.8 mm x 0.8 mm)
0805 (2 mm × 1.25 mm × 1.25 mm)
VOLTAGE RATING
6.3 V
10 V
6.3 V
10 V
8.2.2.3 Inductor Selection
The LM3556 is designed to use a 1-µH or 0.47-µH inductor. Table 17 lists various inductors and their
manufacturers that work well with the LM3556. When the device is boosting (V_OUT > V_IN) the inductor is typically
the largest area of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible series
resistance is important. Additionally, the saturation rating of the inductor must be greater than the maximum
operating peak current of the LM3556. This prevents excess efficiency loss that can occur with inductors that
operate in saturation. For proper inductor operation and circuit performance, ensure that the inductor saturation
and the peak current limit setting of the LM3556 are greater than I_PEAK in Equation 3:
I_PEAK
=
I_LOAD
K
x
V_OUT
V_IN
+ 'I_L
where
'I_L =
V_IN x ⁽V_OUT - V_IN⁾
2 x f_SW x L x V_OUT
where
• ƒ_SW = 4 MHz
• Efficiency can be found in Typical Characteristics.
(3)
MANUFACTURER
TOKO
TOKO
TOKO
L
1 µH
1 µH
1 µH
Table 17. Recommended Inductors
PART NUMBER
FDSD0312
DFE252010C
DFE252012C
DIMENSIONS (L × W × H)
3 mm x 3 mm x 1.2 mm
2.5 mm × 2 mm × 1 mm
2.5 mm × 2 mm × 1.2 mm
I_SAT
4.5 A
3.4 A
3.8 A
R_DC
43 mΩ
60 mΩ
45 mΩ
28
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